Method and apparatus for charged particle spectroscopy

ABSTRACT

In a cylindrical mirror analyzer, charged, particles from a source are deflected in the radial electric field between a pair of coaxial tubular electrodes, and are brought to a focus. A collector aperture positioned at the focus selects particles of predetermined energy, these particles being detected by a suitable detector. The particles cross the axis at some point between the source and the focus. An apertured member located at this point prevents skew electrons (i.e. electrons not travelling in radial planes) from reaching the focus. In this way the performance of the analyzer is improved.

United States Patent [191 Watson METHOD AND APPARATUS FOR CHARGEDPARTICLE SPECTROSCOPY [75] Inventor: John Merza Watson, Manchester,

England [73] Assignee: Associated Electrical Industries Limited, London,England [22] Filedf Mar. 21, 1972 [2]] Appl. No.: 236,748

[30] Foreign Application Priority Data Mar. 23, 1971 Great Britain7,594/71 52 0.5. c: 250/305, 250/282, 250/294 [51] Int. Cl. H0lj 39/34[58] Field of Search 250/495 AE, 49.5 PE, 250/419 ME [56] ReferencesCited UNITED STATES PATENTS 3,699,331 10/1972 Palmberg 250/495 Jan.1,1974

3,617,741 11/1971 Siegbahn et a1. 250/495 3,596,091 7/1971 Helmer et al.250/495 3,609,352 9/1971 Harris 250/495 Primary Examiner-William F.Lindquist Att0mey-Thomas E. Fisher et a1.

[57] ABSTRACT In a cylindrical mirror analyzer, charged, particles froma source are deflected in the radial electric field between a pair ofcoaxial tubular electrodes, and are brought to a focus. A collectoraperture positioned at the focus selects particles of predeterminedenergy, these particles being detected by a suitable detector. Theparticles cross the axis at some point between the source and the focus.An apertured member located at this point prevents skew electrons (i.e.electrons not travelling in radial planes) from reaching the focus. Inthis way the performance of the analyzer is improved.

19 Claims, 6 Drawing Figures PATENTEU 1 I974 SHEET 1 [1F 2 PATENTEU 1I974 Fig.5

SHEET 2 UF 2 Fig.6

METHOD AND APPARATUS FOR CHARGED PARTICLE SPECTROSCOPY CROSS REFERENCETO RELATED APPLICATION Electron Spectroscopy, US. Patent applicationSer. No. 119,327, filed Feb, 26,1971, by Brian Noel Green and John MerzaWatson.

BACKGROUND OF THE INVENTION This invention relatesto charged particlespectroscopy and relates particularly, although not exclusively, tomethods and apparatus for use in electron spectroscopy.

In the art of charged particle spectroscopy, devices called cylindricalmirror analyzers are known which analyze the energy spectrum of theparticles by injecting the particles into a radial field producedbetween a pair of coaxially mounted tubular electrodes held at differentpotentials. Charged particles injected into the radial electric fieldbetween the tubular electrodes are deflected by the fieldtoward the axisof the electrodes. Particles of. a predetermined energy are therebybrought to a focus. By positioning a collector aperture at this focus,particles of a predtermined energy are selected by the aperture anddetected as they pass through the aperture. By sweeping the voltageacross the electrodes through a range of values, and detecting as afunction of time such particles as pass through the collector aperturethe energy spectrum of the injected particles is obtained.

Undesirable aberrations will be introduced where the source of chargedparticles islarger than a single point disposed along the axis of theanalyzer. By way of explanation, if the source region is effectively apoint disposed on the analyzer axis, all particles which emergetherefrom will travel in planes containing the axis of the analyzer.This is to say the particles will all travel in planes which are radialto the axis of the analyzer. As such, particles of the same energy willall be focused by the radial field at a .common focus which is likewisealong the analyzer axis.

Where the source is not effectively a point, some of the particlesintroduced into the radial field will be caused to travel in paths whichare askew, i.e., along paths which are not confined to a single radialplane. These skew particles will not be focused at the same place asnon-skew particles of the same energy. Accordingly undesirableaberrations are introduced into the analyzer thereby reducing theresolving power of the instrument.

Although axial point sources are desirable in that they introduce noskew particles, such sources have a very small area and provide a verylow sensitivity analy- SIS.

It is known that sensitivity can be improved, at least to some extend,by shifting the source position from the axis so that the source becomesan annulus, or partannulus, about the axis. This gives rise to anincrease in source area and hence to a large increase in sensitivity.However, it also leads to a decrease in resolving power, due to theeffect of skew or non-radial particle trajectories.

SUMMARY OF THE INVENTION According to the invention, in a cylindricalmirror analyzer, said detector is so positioned that, in operation,particles of said predetermined energy from the source region, travelingin radial planes, cross the axis of the analyzer at at least'oneposition before reaching the detector. A screen member having anaperture, hereinafter referred to as the B-aperture, is disposed at thisone position so asto prevent at least the majority of particlestraveling in skew paths from reaching the detector.

Preferably, said collector aperture is positioned offaxis with respectto the analyzer axis. In this way, the collector aperture does notinterfere with the function of the B-aperture Conveniently, thecollector aperture substantially coincides with the inner of theelectrodes.

Preferably, the B-aperture is disposed at a position reached by theparticles after being deflected by the radial field.

In one preferred arrangement in accordance with the invention, theanalyzer is so arranged that, in operation, the particles, after passingthrough the fl-aperture, re-enter the radial electric field, and aredeflected again by the field towards the detector, which is positionedon the axis. e

Preferably, the particles from the source region pass through a furtheraperture, hereinafter referred to as the a-aperture, which restricts theangle of divergence of the paths of those particles which reach thedetector. The a-aperture preferably is positioned off-axis with respectto the analyzeraxis, so that it does not interfere with the function ofthe B-aperture, and is preferably positioned upstream of said collectoraperture. Conveniently, the a-aperture substantially conicides with theinner of the electrodes.

Preferably, the collector aperture is annular and disposed symmetricallywith respect to the analyzer axis. Such an annular configuration allowsthe sensitivity of the analyzer to be optimised for a given resolvingpower.

Conveniently, there is provided means selectably op erable to retard theparticles before they enter the radial field. A more complete discussionof electron retardation is provided in the referenced application. Suchretardation enables the sensitivity of the analyzer to be improvedwithout a corresponding loss of resolving power, as will be described.The means for retarding the particles conveniently comprises a chargedparticle lens. j

.7 Accordingly, one object of the present invention is to provide acylindrical mirror analyzer in which aberrations associated with skewparticles are reduced.

BRIEF DESCRIPTION OF THE DRAWINGS.

This and other objects and advantages of the invention will become moreapparent from the following description of preferred embodiments of theinvention as. read in conjunction with the accompanying drawings,comprising FIGS. 1-6, each of which is a schematic sectional elevational view of a cylindrical mirror analyzer in accordance with theinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS.

A specimen 3 is mounted on a specimen support within the inner spectrode1, on the common axis 4 of the electrodes. In operation of the analyzer,the specimen 3 is bombarded with radiation, such as, for example,X-rays, ultraviolet radiation, or electrons from a radiation source 14so as to cause the specimen to emit electrons. The energies of theseemitted electrons will depend on the chemical structure of the sample,and on the nature of the bombarding radiation. The specimen 3 issurrounded by a screening member 5, having a small aperture 6 throughwhich the electrons emitted from the specimen pass, which aperture thuseffectively constitutes a point source of electrons.

After they pass through the source aperture 6, the electrons passthrough a further aperture 7, which in this case is formed in the innerelectrode 1 but may be formed in a separate member, this aperture 7serving to restrict the angle of divergence 2a of the paths of thoseelectrons from the source aperture 6 which pass through the aperture 7.The aperture 7 is therefore referred to as the oz-aperture. Electronspassing through the a-aperture 7 enter the radial electric field betweenthe electrodes 1 and 2 and are deflected by the field towards the axis 4of the analyser. The deflected electrons pass through an exit aperture 8in the inner electrode 1, the exit aperture 8 being sufficiently wide asto have no limiting effect on the electrons. The electron optics of acylindrical mirror analyser are such that electrons of a predeterminedenergy (depending on the strength of the radial electric field) andtravelling in radial planes with respect to the analyser, are brought toa focus at a point 9. The focus is a second order one; i.e. the focusingof the electrons at the point 9 is independent of terms in a and a theprincipal aberration being proportional to a A collector aperture 10 isp0- sitioned at this point to select electrons of the predeterminedenergy. Electrons passing through this aperture enter an electrondetector 11, which produces an output signal proportional to the rate atwhich it receives electrons.

It will be appreciated that the resolving power of the analyser (i.e.its ability to discriminate between electrons of different energies)will depend, among other things, on the extent of the collector aperture10, in the axial direction, and on the extent of the source aperture 6,in the axial direction. The. smaller these apertures are, the greaterthe resolving power. However, reducing the sizes of these apertures willalso reduce the sensitivity of the analyser, so that, in practice, thesizes of the apertures must be a compromise.

The radial position of the focus 9, i.e. its radial distance 1 from theinner electrode 1, depends on 1 the radial distance of the sourceaperture 6 from the inner electrode 1, and also on the mean angle 6which the trajectories of the electrons make with the axis 4 before theelectrons are injected into the radial electric field. For a given angle0, the distances l and 1 are related by the equation:

l +l =nn where n is a constant, dependent on 6. In the present example,I +1 2r i.e. n 2, which corresponds approximately to an angle 0 4220.

It will be seen that, with this geometry, the electrons will cross theanalyser axis 4 at some position between the exit aperture 8 and thefocus 9. This applies only to electrons travelling in radial planes,however, and skew electrons (i.e. those not travelling in radial planes)will not cross the axis 4, the distance of their closest approach to theaxis being a direct measure of their skewness (i.e. the angle ,8 bywhich the diverge from a radial plane). A major proportion of these skewelectrons is excluded from reaching the detector 11 by means of acircular aperture 12 (hereinafter referred to as the B-aperture) locatedon the axis 4 at the position where the non-skew electrons crossthe-axis.

Clearly, the smaller the B-aperture 12 the more effective it is inexcluding the skew electrons, although of course the smaller it is thesmaller the number of electrons which will reach the detector 11 andhence the lower the sensitivity of the analyser. In practice, the sizeof the ,B-aperture 12 will be a compromise. The effect of the,B-aperture 12 is greatly to decrease aberrations due to skew electrons,and thus toimprove the performance of the analyser.

Because the source aperture 6, a-aperture 7 and the collector aperture10 lie off-axis with respect to the axis 4, they do not interfere withthe function of the ,B-aperture 12. If, for example, the collectoraperture 10 were positioned on the axis, it would, in general, restrictthe flow of non-axial electrons more severely than is necessary toachieve the required resolution, and would thus unnecessarily reduce thesensitivity of the analyser.

The energy spectrum of the electrons emitted from the sample can beanalysed by sweeping the voltage applied between the electrodes 1 and 2through a suitable range of values, and recording the output of theelectron detector 11 as a function of time.

Referring now to FIG. 2, in which similar features have the samereference numerals as in FIG. 1, in a modification of the arrangement ofFIG. 1 the source aperture 6, the a-aperture 7, the exit aperture 8, thefocus 9 the collector aperture 10, and the detector 11 are all annularin form extending through an azimuthal angle of 2 1r coaxially with theaxis 4. The electron optics of this arrangement aresimilar to those ofthe arrangement of FIG. 1. However, it will be seen that in this case,the area of the source aperture 6 is much greater than in the case ofFIG. 1, and thus the sensitivity of the analyser is greatly increased.As before, the analyser comprises a circular B-aperture 12 at the pointwhere the non-skew electrons across the axis.

In other modifications of the arrangement of FIG. 1, the arrangement maybe only part annular, extending through an azimuthal angle of less than2 11.

In modifications of the arrangements shown in FIGS. 1 and 2, if thedistances 1, and 1 are both varied, while keeping the sum 1 1 constant,the factor n in equation 1 above will remain constant, and therefore theangle 6 required for second order focussing at the collector aperturewill remain the same, i.e. approximately equal to 42 20'. Furthermore,the aberration coefficients will remain the same, and the dispersionexpression (i.e. the relation between electron energy and appliedvoltage) for the analyser will be retained. A limiting case of this isshown in FIG. 3, for the case where l,= O and therefore 1 =2r Referringto FIG. 3, the analyser shown therein comprises a pair of coaxialtubular electrodes 21 and 22 for producing a radial electric field. Aspecimen 23 is positioned by means of a specimen support 35 on theanalyser axis 24, and is arranged'to be bombarded with radiation from aradiation source 34 and thus to emit electrons. The emitted electronspass through an annular aperture 25 in the inner electrode 21, thisaperture thus constituting effectively an annular source of electrons. Afurther annular aperture 26 is positioned between the specimen 23 andthe source aperture 25, and serves as an a-aperture, defining the angleof divergence 2 a of the electrons passing through the source aperture.

Electrons from the source aperture 25 enter the radial electric fieldbetwen the electrodes 21 and 22, and are deflected towards the axis 24,passing through an annular exit aperture 27 (which is large enough tohave no limiting effect on the electrons) in the inner electrode 21.Electrons of a predetermined energy are thus brought to an annularsecond order focus 28. Since in this case l 2r the focus 28 lies on theinner electrode 21. An annular aperture 29 is formed in the innerelectrode 21, and serves as a collector aperture, selecting cause.

In a modification of the arrangement shown in FIG.

3, the aperture 27 may be arranged to act as the a-aperture, effectivelylimiting the divergence of the electron beam. In this case the aperture26 will be chosen to be large enough to have no limiting effect on theelectrons, or may be omitted altogether, thus leaving more working spacearound the specimen 23.

In other modifications of the arrangements shown in FIGS. 1 and 2, thedistances 1, and 1 may be modified independently of each other, so thatthe factor n in equation (1) no longer remains equal to 2. It will beappreciated that, in such an arrangement, in order to achieve secondorder focussing, the angle 0 must be other than 42 One such modificationis shown in FIG. 4, which shows a cylindrical mirror analysercomprising, as before, a pair of coaxial tubular metal electrodes 41 and42, and an annular source 43 of electrons, whichmay comprise a sourceaperture such as aperture 6 in FIGS. 1 and 2. Electrons from the source43 pass through an annular a-aperture 44 in the inner electrode 41, aredeflected by the radial electric field between the electrodes towardsthe analyser axis 45 through an annular exit aperture 46 (having nolimiting effect on the electrons) in the inner electrode, electrons of apredetermined energy and travelling in radial planes being brought to asecond order focus 47. In the particular example shown, the angle 0 andthe distance I, are so chosen that this focus 47 is a point focus, lyingon the axis 45. However, in other arrangements, this focus could beannular or part annular. A circular collector aperture 48 is disposed atthe focus 47, and selects focused electrons having the predeterminedenergy. The selected electrons pass into an electron detector 49.

It will be seen that in this arrangement, the electrons cross the axis45 at a point between the source 43 and the a-aperture 44 i.e. beforethey enter the radial electric field. A circular B-aperture 50 islocated on the axis at this point, in order to prevent the majority ofskew electrons from reaching the detector 49.

In the particular example shown in FIG. 4, it will be seen that 1 isgreater than r, and I =r,, so that the factor n in equation (1) isgreater thus 2. For example, if 1, =1 .5r n=2.5

Referring to FIG 5, a preferred analyser in accordance with theinvention comprises a pair of coaxial tubular electrodes 62 and 63 forproducing a radial electric field. A specimen 60 is positioned by meansof a specimen support on the analyser axis 64, and is arranged to bebombarded with radiation from a radiation source 74 and thus to emitelectrons. The emitted electrons pass through an annular source aperture61 in the inner electrode 62, this aperture thus effectivelyconstituting an annular source of electrons.

Electrons from the source aperture 61 enter the radial electric fieldbetween the electrodes 62 and 63, and are deflected towards the axis 64.The electrons pass through an annular aperture 65 in the inner electrode62, this aperture serving as an a-aperture defining the effective angleof divergence 2a of the electrons from the source aperture 61.

The radial electric field focuses electrons of a predetermined energy toan annular second order focus 66, which lies coincident with the innerelectrode 62. An annular collector aperture 68 in the inner electrodeselects focussed electrons of the predetermined energy.

The electrons cross the analyser axis 64 at a point intermediate thea-aperture 65 and the focus 66, and a circular ,B-aperture is positionedon the axis of that point, so as to exclude the majority of skewelectrons and therefore reduce the aberrations from this cause.

As described so far, the arrangement of FIG. 5 is similar to that ofFIG. 3. However, in FIG. 5, the electrons do not enter an annulardetector after passing through the collector aperture 68. Instead, theelectrons reenter the radial electric field between the electrodes 62and 63, and are deflected by the field again towards the axis 64, sothat they pass through an exit aperture 69 in the inner electrode 62.The electrons converge on to, and pass through, a circular detectoraperture 71, on the axis 64, and are deflected by a tubular electrode 72into an electron detector 70. Neither the exit aperture 69 nor thedetector aperture has any limiting effect on the electrons.

It will be seen that by passing the electrons through the radial fieldfor a second time, the electronscan be directed on to a relativelylocalised region, so that a single detector can be used. Thisavoids theuse of an annular detector, as required in FIGS. 2 or 3.

In a modification of the arrangement of FIG. 5, the collector aperturemay be positioned where the electrons emerge from the radial field forthe second time ie may replace exit aperture 69.

In the arrangements described above the various apertures, such as theapertures 61, 65 and 68 in FIG. 5 are defined by the inner electrode ofthe analyser. However, in modifications of these arrangements, theapertures may be defined by separate plates, which are mounted adjacentthe inner electrode. This allows the apertures to be replaced if it isdesired to change their dimensions. Such separate plates may be held atthe same potential as the inner cylinder, or may be held at a slightlydifferent potential, such as to effect small corrections to theinjection angle 0 and to compensate for the effect of fringing fields atthe apertures.

In the particular embodiments described above the effective source ofelectrons is defined by an aperture,

such as the aperture 6 in FIG. 1. However, in modifications of thesearrangements, the effective source may be the area of the surface of aspecimen after which the electrons actually originate. Alternatively,the apparatus may include an electron optical lens, arranged to bringelectrons to a focus, (which may be a point focus, or an annular or partannular focus), which focus then constitutes the effective source ofelectrons for the apparatus. The electron optical lens, in addition tofocusing the electrons, may also retard them ie decrease their energies.The advantage of retarding electrons in apparatus for electronspectroscopy 'is, as explained in I the referenced application, that incertain circumstances it allows the sensitivity of the apparatus to beincreased, at least for relatively high energy electrons, without lossof resolving power.

Referring now to FIG. 6, in one such embodiment of the invention thereis again provided a cylindrical mirror analyser comprising two tubularelectrodes 80 and 81 disposed coaxially with respect to a common axis82, between which a radial electric field can be produced. A specimen 83is mounted in a specimen support 96 on the axis 82, and is arranged tobe bombarded with radiation from a radiation source 94 so as to emitelectrons. These electrons pass through an annular source aperture 84 inthe inner electrode 80, are deflected towards the axis 82 by the radialelectric field and pass through an annular a-aperture 85 in the innerelectrode. Electrons of a predetermined energy are thus brought to anannular focus 86 coincident with the inner electrode 80. Electrons ofthis predetermined energy are selected by a collector aperture 95 formedin the inner electrode 80. Electrons selected by the collector aperture95 either enter an annular electron detector (not shown) as in the caseof FIG. 3, or pass for a second time through the radial field and areconverged on to a detector (not shown) on the axis 82, as in the case ofFIG. 6. As before, a ,B-aperture 87 is positioned at a point where theelectrons cross the axis 82 before arriving at the focus 86, in order toprevent the majority of skew electrons from reaching the focus 86.

Between the specimen 83 and the aperture 84 is positioned an electronoptical lens arrangement 88, comprising three metal components, 89, 90,91 each of which is in the shape of a truncatedcone coaxial with theelectrodes 80, 81, and each of which has an annular aperture 92 throughwhich the electrons pass successively. It will be seen that by applyingsuitable potentials to the components 89, 90, 91, the lens arrangement88 can be made to focus electrons emitted from the specimen 83 to anannular focus 93 at the aperture 84, and may also be arranged to retardthe electrons before they enter the radial field. Thus, it will be seenthat this focus 93 acts effectively as an annular source of electrons.

The lens arrangement is selectably operable by changing the electricalconnections thereto, either to produce or not to produce retardation,and either to produce a substantially unit electron opticalmagnification, or to produce an electron optical magnificationsubstantially greater than unity. Retardation is produced by setting thepotentials of components 89, 90,

91 such that the potential of the last component 91 is ergy electrons,for the reason explained above, while no retardation is used foranalysing relatively low energy electrons.

A magnification greater than unity is used in one of two circumstances.First, where the specimen is irradiated with a fine probe of radiation,(i.e. a microprobe) the area of the specimen which emits electrons isvery small, and the use of a magnification substantially greater thanunity allows a greater number of the electrons emitted from this smallarea to pass through the analyser, thereby increasing the sensitivity ofthe analyser. Secondly, where the specimen is flooded with radiation,the use of a magnification substantially greater than unity means thatthe electrons which pass through the source aperture 84 come only from avery small area of the specimen. This latter arrangement provides ineffect a virtual microprobe and can be used in a similar manner toconventional microprobe arrangements.

A substantially unit magnification, on the other hand, is used if thesample were flooded with radiation to cause it to emit electrons over anextended area. v

It will be appreciated that in modifications of the arrangement shown inFIG. 6 the lens 88 may comprise more than three components or mayalternatively comprise only two components. Furthermore the componentsof the lens may have substantial thickness, so that the apertures 92become channels of significant length.

It should be appreciated that while the particular embodiments of theinvention described herein, by way of example, are for electronspectroscopy, the invention is equally applicable to other forms ofcharged particle spectroscopy, e.g. ion spectroscopy.

Although the invention has been shown in connection with preferredembodiments it will be readily apparent to those skilled in the art thatvarious changes in the form and arrangement of parts may be made to suitrequirements without departing from the spirit and scope of theinvention as defined by the appended claims.

I claim:

1. A cylindrical mirror analzer comprising:

a pair of at least partially tubular electrodes mounted coaxially withrespect to a common axis and mutually electrically insulated so thatwhen a voltage is applied between said electrodes a radial electricalfield is established between said electrodes;

source means for injecting charged particles into said radial fieldwhereby particles emanating from said source means are deflected by saidfield toward said axis and particles of a predetermined energy aredirected toward a focal point;

a first apertured meanshaving a collector aperture formed therein andpositioned at said focal point to permit passage of substantially onlysaid particles of predetermined energy;

a detector means for detecting particles which pass through saidcollector aperture; and,

a second apertured means having an aperture formed therein andpositioned at a point where said particles of said predetermined energywhich are traveling in radial planes with respect to said analyzer axiscross said analyzer axis.

2. A cylindrical mirror analyzer according to claim 1 wherein saidcollector aperture is positioned off-axis with respect to the analyzeraxis.

3. A cylindrical mirror analyzer according to claim 2 vwherein saidcollector aperture substantially coincides in diameter with the diameterof the inner of said electrodes.

4. A cylindrical mirror analyzer according to claim 2 wherein saidcollector aperture is annular and disposed symmetrically with respect tosaid axis.

5. A cylindrical mirror analyzer according to claim 1 wherein saidsecond apertured means is positioned between said source means and saidcollector aperture.

6. A cylindrical mirror analyzer according to claim 5, wherein saidsecond apertured means is disposed at a position reached by saidparticles after being deflected by said field.

7. A cylindrical mirror analyzer according to claim 6, wherein, saiddetector means is positioned along said axis and said particles, afterpassing through said second apertured means, re-enter the radial fieldand are deflected again by the field towards said detector means.

8. A cylindrical mirroranalyzer according to claim 1, further includinga third apertured means having an aperture formed therein forrestricting the angle of divergence from said source means of the pathsof those particles which reach said detector means.

9. A cylindrical mirror analyzer according to claim 8 wherein saidaperture in said third apertured means is positioned off-axis withrespect to said analyzer axis.

10. A cylindrical mirror analyzer according to claim 9 wherein saidaperture in said third apertured means substantially coincides indiameter with the diameter of the inner of said electrodes.

11. A cylindrical mirror analyzer according to claim 8 wherein saidthird apertured means is positioned upstream of said collector aperture.

12. A cylindrical mirror analyzer, according to claim 1 furtherincluding means selectably operable to retard said particles before theyenter said field.

13. A cylindrical mirror analyzer according to claim 12, wherein saidmeans to reatrd the particles com- 10 prises a charged-particle lens.

14. A cylindrical mirror analyzer according to claim 13, wherein saidlens is an electrostatic lens comprising a plurality of metal componentshaving respective apertures through which the particles passsuccessively.

15. A cylindrical mirror analyzer according to claim 14, wherein saidmetal components are generally conical in shape.

16. A method of operating a cylindrical mirror analyzer having a pair oftubular electrodes mounted coaxially with respect to a common axis andmutually electrically insulated, comprising the steps:

a. injecting charged particles between said electrodes;

b. applying a voltage between said electrodes so as to deflect saidparticles towards said axis thereby bringing particles of apredetermined energy to a focus; V

c. selecting said particles of said predetermined en-' ergy at saidfocus;

d. positioning a member having; an aperture on said axis at a said saiparticles of predetermined energy, travelling in radial planes, crosssaid axis, so as to prevent at least the majority of particlestravelling in skew paths from reaching said focus; and

e. detecting the particles so selected.

17. The method of claim 16 wherein said step of selecting particles iscarried out by inserting an apertured means having a collector apertureformed therein at said focus.

. 18. The method of claim 16, including the further step of retardingsaid particles before they enter said field.

19. The method of claim 18,; wherein said step of re tarding saidparticles is carried out by operating a charged-particle lens. I

1. A cylindrical mirror analyzer comprising: a pair of at leastpartially tubular electrodes mounted coaxially with respect to a commonaxIs and mutually electrically insulated so that when a voltage isapplied between said electrodes a radial electrical field is establishedbetween said electrodes; source means for injecting charged particlesinto said radial field whereby particles emanating from said sourcemeans are deflected by said field toward said axis and particles of apredetermined energy are directed toward a focal point; a firstapertured means having a collector aperture formed therein andpositioned at said focal point to permit passage of substantially onlysaid particles of predetermined energy; a detector means for detectingparticles which pass through said collector aperture; and, a secondapertured means having an aperture formed therein and positioned at apoint where said particles of said predetermined energy which aretraveling in radial planes with respect to said analyzer axis cross saidanalyzer axis.
 2. A cylindrical mirror analyzer according to claim 1wherein said collector aperture is positioned off-axis with respect tothe analyzer axis.
 3. A cylindrical mirror analyzer according to claim 2wherein said collector aperture substantially coincides in diameter withthe diameter of the inner of said electrodes.
 4. A cylindrical mirroranalyzer according to claim 2 wherein said collector aperture is annularand disposed symmetrically with respect to said axis.
 5. A cylindricalmirror analyzer according to claim 1 wherein said second apertured meansis positioned between said source means and said collector aperture. 6.A cylindrical mirror analyzer according to claim 5, wherein said secondapertured means is disposed at a position reached by said particlesafter being deflected by said field.
 7. A cylindrical mirror analyzeraccording to claim 6, wherein, said detector means is positioned alongsaid axis and said particles, after passing through said secondapertured means, re-enter the radial field and are deflected again bythe field towards said detector means.
 8. A cylindrical mirror analyzeraccording to claim 1, further including a third apertured means havingan aperture formed therein for restricting the angle of divergence fromsaid source means of the paths of those particles which reach saiddetector means.
 9. A cylindrical mirror analyzer according to claim 8wherein said aperture in said third apertured means is positionedoff-axis with respect to said analyzer axis.
 10. A cylindrical mirroranalyzer according to claim 9 wherein said aperture in said thirdapertured means substantially coincides in diameter with the diameter ofthe inner of said electrodes.
 11. A cylindrical mirror analyzeraccording to claim 8 wherein said third apertured means is positionedupstream of said collector aperture.
 12. A cylindrical mirror analyzer,according to claim 1 further including means selectably operable toretard said particles before they enter said field.
 13. A cylindricalmirror analyzer according to claim 12, wherein said means to retard theparticles comprises a charged-particle lens.
 14. A cylindrical mirroranalyzer according to claim 13, wherein said lens is an electrostaticlens comprising a plurality of metal components having respectiveapertures through which the particles pass successively.
 15. Acylindrical mirror analyzer according to claim 14, wherein said metalcomponents are generally conical in shape.
 16. A method of operating acylindrical mirror analyzer having a pair of tubular electrodes mountedcoaxially with respect to a common axis and mutually electricallyinsulated, comprising the steps: a. injecting charged particles betweensaid electrodes; b. applying a voltage between said electrodes so as todeflect said particles towards said axis thereby bringing particles of apredetermined energy to a focus; c. selecting said particles of saidpredetermined energy at said focus; d. positioning a member having anaperture on said axis at a said particles of predetermined energy,travelling in radial planes, cross said axis, so as to prevent at leastthe majority of particles travelling in skew paths from reaching saidfocus; and e. detecting the particles so selected.
 17. The method ofclaim 16 wherein said step of selecting particles is carried out byinserting an apertured means having a collector aperture formed thereinat said focus.
 18. The method of claim 16, including the further step ofretarding said particles before they enter said field.
 19. The method ofclaim 18, wherein said step of retarding said particles is carried outby operating a charged-particle lens.